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Abstract

We investigate the phase noise performance of micro- and millimeter-wave signals generated using a ultra-high Q whispering gallery mode disk-resonator with Kerr nonlinearity. Our study focuses on the stability of the optical spectra and on the performances of the corresponding microwave and millimeter-wave beat notes in terms of power and phase noise. The blue slope of an optical mode of the resonator, allowing for the generation of optical frequency combs, is accurately explored in order to identify various comb patterns. Each of these patterns is characterized in the optical and radio-frequency domains. Phase noise levels below −100 dBc/Hz at 10 kHz offset have been achieved for beat notes in the radio-frequency spectrum at 12 GHz, 18 GHz, 24 GHz, 30 GHz, and 36 GHz with the same resonator.

Figures (7)

Fig. 1 (a) MgF2 WGMR fixed on a specific mount and coupled to (one or two) tapered optical microfibers. Two mirrors facing each other are used to get an accurate side view of the microfibers position regarding the WGMR rim. (b) Illustration of (Degenerate) Four-Wave-Mixing [(D)FWM] for Kerr comb generation.

Fig. 3 Normalized absorption signal recorded at the slow photodiode output by an oscilloscope for the MgF2 WGMR, where different excited optical modes can be seen. The 20 ms time scale on this graphic represents a 1.3 GHz laser frequency scanning range (visualization window). The inset figure shows the WGMR mode that has been used in our experiment and accurately investigated using the side-of-fringe locking technique in order to identify the different generated Kerr comb patterns. The various laser-mode locking states that has been considered are also shown (S1, S2, etc.). The red line draws the locking limit imposed by the laser frequency noise combined to the WGMR thermal instabilities, and also to the limits of the side-of-fringe locking scheme.

Fig. 4 Measured optical spectra at the drop port of the WGMR and their corresponding RF spectra, obtained for different Kerr comb patterns generated at different laser-mode locking states (see Fig. 3). The measured single sideband phase noise spectra of the Kerr combs’ beat note at 18 GHz are given for locking states S1, S2, S4 and S6 with the corresponding carrier RF power. For state S4, the phase noise spectra of both beat notes at 18 GHz and 36 GHz are given and compared. (OSA: RBW = 12.5 MHz; ESA: RBW = 50 kHz and VBW = 50 kHz).

Fig. 6 Comparison of the Kerr comb performances at the WGMR drop port between states S4 and S5 [see the inset figure in Fig. 3 and Fig. 4]. (a), (b) and (c): RF spectra recorded for states S4 and S5 and centered at beat notes 6 GHz (only for S5), 18 GHz and 36 GHz, respectively. (d) Optical spectrum, (e) low frequency DC spectrum and (f) Large span RF spectrum recorded for the Kerr comb generated at states S5 and compared with state SA spectra where the laser lightwave was slightly resonant but no Kerr comb was generated [see the inset figure in Fig. 3].

Fig. 7 Comparison of the performances of different Kerr combs (A–F) generated at the through port using different optical modes of the MgF2 WGMR. (A–F) represent the optical spectra for the different combs. (a) Comparison of the wide span RF spectra of the different combs. (b) Comparison of the phase noise spectra of the main beat notes (i.e. 6–36 GHz, for the spectra A–F, respectively).